When producing cellulose-based products, a roll press is frequently present for dewatering the cellulose-based pulp. The pulp is passed between two cooperating press rolls in a roll press, the press rolls having a perforated envelope surface, whereby the envelope surface is permeable to liquid pressed out of the pulp, and is pressed in the roll nip, or the press nip, between the press rolls, whereby liquid is pressed out of the pulp. One example of such a roll press for dewatering pulp is disclosed in Swedish Patent No. 505,539, e.g., where the central axes of the press rolls lie in substantially the same horizontal plane, and the pulp passes the press nip between the press rolls in a vertical direction, from below upwards.
Swedish Patent No. 519,753 describes the sealing of a press roll in a roll press having two cooperating press rolls for dewatering material suspensions.
The above mentioned roll presses have in common that their roll bodies have a conventional structure, where the roll body comprises a longitudinal central drum, which can comprise a drum or an inner drum covered with a covering plate, or the like. The central drum is provided with several supporting ribs arranged longitudinally along the longitudinal axis of the central drum and evenly distributed along the circumference of the central drum. The purpose of these supporting ribs is to strengthen the structure, and also to provide channels which axially guide liquid pressed out of the pulp to the ends of the press roll. On these supporting ribs, a mantle section is attached, comprising several circumferential frame rings which are surrounding the roll body along its entire length, the purpose of which is to be a supporting structure for the perforated roll plates which are positioned outermost on the press roll, since the roll plates are abutting the frame rings around their whole circumference, and to allow expressed liquid, which has passed through the roll plate, to fall downwards after the passage through the apertures of the roll plate. This is important in order to reduce the rewetting, i.e. the liquid which, after the press nip, flows back through the roll plate and dilutes the pressed pulp, especially for roll presses where the pulp is passed through the press nip between two press rolls in a vertical direction from below upwards, such as in Swedish Patent No. 505,539. The result of this rewetting is that the resulting dryness of the pressed pulp from the roll press is lower, and that the pulp is contaminated by lye or liquid previously pressed out.
U.S. Pat. No. 6,311,849, upon which the preamble of claim 1 is based, discloses an apparatus for dewatering and washing pulp, comprising two press rolls, between which the pulp is passed in a vertical direction from below upwards, the rotation axes of the press rolls lying on a horizontal plane. The press rolls comprise several pairs of chambers arranged longitudinally along the longitudinal axis of the central drum and between the central drum and the roll plate, the chambers of a pair being separated from each other, and the first chamber of the pair comprises inlets for liquid, which defines a flow path in a substantially radial direction, whereas the second chamber of the pair comprises at least one inflow opening for the inflow of liquid, which define a flow path in a substantially tangential direction. The second chamber of a pair is in direct communication, by means of its inflow opening, with the first chamber of an adjacent pair of chambers for the inflow of liquid from the first chamber. From these chambers, the liquid from the pulp is then axially removed. By this, the rewetting of the pulp is reduced.
One object of the present invention is thus to attain a roll body for a press roll which provides a press roll which, in use in a roll press, is more effective in washing the pulp and provides the required purity of the washed pulp. Further, one object is to attain a roll press for washing pulp comprising press rolls provided with such a roll body.